You need only to change the page "include:language-flags"
- it is used only in the :_template page of every category included at the end ( have a look here on the "forum:_template" how the flags and navigaion is inserted…)

done with [[include include:language-flags pagename=start]]

and for the start page of the handbook we use the horizontal flags:include:language-flags-h
[[include include:language-flags-h pagename=start]]

in all other pages ( like en:xxx-xxx-xxx) the _template page holds another call:
[[include include:language-flags pagename=%%page_name%%]] - where the "actual" pagename is inserted